1. Field of the Invention
This invention relates to a resin laminate comprising a layer of a specific modified ethylene/vinyl acetate copolymer resin and a layer of a selected specific thermoplastic resin.
2. Background Art
Having the properties of being stable to heat, excelling in moldability and processability and being of low hygroscopicity, tasteless/odorless and chemically inert, styrene resins have so far been widely used for food containers, packaging materials, goods such as toys and writing utensils, parts of cars or electric appliances for home service, etc.
However, they have the disadvantages of being likely to be attacked by organic solvents, fragile and of insufficient heat resistance. For use as food containers in particular, resins having improved oil resistance, heat resistance and heat sealability are thus in great demand. In order to meet such demand, it has been attempted to laminate polyolefins such as polyethylene and polypropylene or other materials on styrene resins.
However, adhesives capable of making an effective bonding of such styrene resins to polyolefins or other materials are still unavailable. Referring to laminates of styrene resins with ethylene/vinyl acetate copolymer resins, by way of example, it has been attempted to introduce into them a larger amount of vinyl acetate in the form of a copolymer with ethylene in expectation of improvements in their adhesion. An increase in the vinyl acetate unit content of said copolymer, however, rather incurs a drop of its mechanical strength, thus failing to achieve a sufficient improvement in adhesion strength.
Laminates of styrene resins with polyolefins modified by grafting .alpha.,.beta.-unsaturated carboxylic acids onto them or containing epoxy groups, ordinarily known as adhesive polyolefins, have also been available; however, they are found to be less than satisfactory in terms of adhesion performance.
Acrylic resins, on the other hand, excel in various properties such as transparency, weather resistance, chemical resistance and electrical insulating quality and are light in weight, tough and of good processability, and so have been widely used as various molding and coating materials.
However, such acrylic resins, esp. methacrylic resins have the disadvantages of being more likely to be injured and more combustible than a competitive inorganic glass. The acrylic resins are also required to have more enhanced heat resistance and impact resistance for use as molding materials.
As one means for meeting such demands, the lamination of acrylic resins with polyolefins such as polyethylene and polypropylene or with other materials is considered effective.
Various adhesive resins have been proposed to bond acrylic resins to other materials. By way of example, ethylene/vinyl acetate copolymer resins having a relatively high vinyl acetate content may be mentioned.
However, the adhesion between acrylic resins and the copolymer resins is still far from satisfactory. In expectation of improvements in such adhesion, the introduction of a larger amount of vinyl acetate in the form of a copolymer with ethylene has been attempted. However, it has been found that the copolymerization of too large an amount of vinyl acetate rather incurs a drop of the mechanical strength of the resulting copolymer, thus failing to achieve appreciable improvements in adhesion strength.
Furthermore, chlorine-containing resins such as vinyl and vinylidene chloride resins have been available.
Generally excelling in various properties such as resistance to solvents, water, acids and alkalis and flame retardancy, vinyl chloride resins have so far been used in wide applications inclusive of films, wire coatings, floor materials, pipings and sheetings. Generally excelling in moistureproofness and gas barrier properties, vinylidene resins have found wide applications in various packaging films and coating moistureproof materials.
However, both the resins are relatively prone to degrade by heat and light and are too hard, and so should be used in combination with suitable stabilizers or plasticizers. In particular, a soft vinyl chloride resin has the disadvantages of being poor in cold resistance, oil resistance and creep resistance due to the plasticizer content being as high as about 30-40%. To make up for such disadvantages, the lamination of these resins with polyolefins such as polyethylene and polypropylene or with other materials is considered effective.
Various adhesive resins have been proposed so as to bond vinyl chloride resins or vinylidene chloride resins to other materials. An ethylene/vinyl acetate copolymer resin having a relatively high vinyl acetate content is one example.
However, the adhesion of vinyl chloride resins or vinylidene chloride resins to such an ethylene/vinyl acetate copolymer resin is still less than satisfactory. In expectation of improvements in this adhesion, the introduction of a larger amount of vinyl acetate in the form of an .alpha.-copolymer with ethylene has been attempted. However, the copolymerization of too large an amount of vinyl acetate rather incurs a drop of the mechanical strength of the resulting copolymer, thus failing to achieve appreciable improvements in the adhesion strength.
Polycarbonate resins are superior in such properties as heat resistance and mechanical strength, but practically is less than satisfactory in terms of chemical resistance and oxygen barrier properties.
In order to eliminate the disadvantages of such resins while making the best use of their advantages, it has been attempted to laminate on them resins having complementary properties, e.g. a polyolefin excelling in chemical resistance or a saponified ethylene/vinyl acetate copolymer excelling in oxygen barrier properties.
In particular, intensive studies have been made in fields of co-extrusion molding and co-injection molding capable of producing laminated structures efficiently and subsequent stretching.
However, any bonding agent to provide an effective bonding of polycarbonate resins to polyolefins or other materials is not developed as yet. Referring to the lamination of polycarbonate resins and ethylene/vinyl acetate copolymers, by way of example, the introduction of a large amount of vinyl acetate in the form of a copolymer with ethylene in expectation of improvements in adhesion rather gives rise to a drop of the mechanical strength of the resulting copolymers, thus failing to achieve appreciable improvements in adhesion strength.
Furthermore, the capability of polyolefins modified by grafting .alpha.,.beta.-unsaturated carboxylic acids onto them or containing epoxy groups--usually known as adhesive polyolefins--to bond to polycarbonate resins is less than satisfactory.
Thermoplastic polyester resins are superior in such properties as heat resistance and mechanical strength, but practically are far from satisfactory in terms of heat sealability and gas permeability.
In order to eliminate the disadvantage of such resins while making the best use of their advantages, it has been attempted to laminate on them resins having complementary properties, e.g. a polyolefin excelling in heat sealability or a saponified ethylene/vinyl acetate copolymer excelling in gas permeability.
In particular, intensive studies have been made in fields of co-extrusion molding and co-injection molding capable of producing laminated structures efficiently and subsequent stretching.
However, any bonding agent to provide an effective bonding of thermoplastic polyesters to polyolefins or other materials is not developed as yet. Referring to the lamination of a polyester and an ethylene/vinyl acetate copolymer, by way of example, the introduction of a large amount of vinyl acetate in the form of a copolymer with ethylene in expectation of improvements in adhesion rather gives rise to a drop of the mechanical strength of the resulting copolymer, thus failing to achieve appreciable improvements in adhesion strength.
Furthermore, the capability of polyolefins modified by grafting .alpha.,.beta.-unsaturated carboxylic acids onto them or containing epoxy groups--usually known as adhesive polyolefins--to bond to thermoplastic polyester is less than satisfactory.